Juan Carlos Cheang-Wong

Controlled anisotropic deformation of Ag nanoparticles by Si ion irradiation

The shape and alignment of silver nanoparticles embedded in a glass matrix is controlled using silicon ion irradiation. Symmetric silver nanoparticles are transformed into anisotropic particles whose larger axis is along the ion beam. Upon irradiation, the surface plasmon resonance of symmetric particles splits into two resonances whose separation depends on the fluence of the ion irradiation. Simulations of the optical absorbance unambiguously show that the anisotropy is caused by the deformation and alignment of the nanoparticles, and that both properties are controlled with the irradiation fluence.

Anisotropic linear and nonlinear optical properties from anisotropy-controlled metallic nanocomposites

High-energy metallic ions were implanted in silica matrices, obtaining spherical-like metallic nanoparticles (NPs) after a proper thermal treatment. These NPs were then deformed by irradiation with Si ions, obtaining an anisotropic metallic nanocomposite. An average large birefringence of 0.06 was measured for these materials in the 300-800 nm region. Besides, their third order nonlinear optical response was measured using self-diffraction and P-scan techniques at 532 nm with 26 ps pulses. By adjusting the incident lights polarization and the angular position of the nanocomposite, the measurements could be directly related to, at least, two of the three linear independent components of its third order susceptibility tensor, finding a large, but anisotropic, response of around 10(-7) esu with respect to other isotropic metallic systems. For the nonlinear optical absorption, we were able to shift from saturable to reverse saturable absorption depending on probing the Au NPs major or minor axes, respectively. This fact could be related to local field calculations and NPs electronic properties. For the nonlinear optical refraction, we passed from self-focusing to self-defocusing, when changing from Ag to Au.

Large optical birefringence by anisotropic silver nanocomposites

A large optical birefringence of oriented Ag nanoellipsoids embedded in silica was measured using an ellipsometric technique. The two main surface plasmon resonances associated with the axes of the ellipsoid were tuned, allowing us to quantify the light transmission through the samples when placed and rotated between crossed and parallel polarizers. This birefringence can be physically associated with the selective optical absorption of one component of the linear polarization of the incident light with respect to the anisotropic axis of the sample, depending on the wavelength used to perform the measurement.

Determination of the size distribution of metallic nanoparticles by optical extinction spectroscopy

A method is proposed to estimate the size distribution of nearly spherical metallic nanoparticles (NPs) from optical extinction spectroscopy (OES) measurements based on Mies theory and an optimization algorithm. The described method is compared against two of the most widely used techniques for the task: transmission electron microscopy (TEM) and small-angle x-ray scattering (SAXS). The size distribution of Au and Cu NPs, obtained by ion implantation in silica and a subsequent thermal annealing in air, was determined by TEM, grazing-incidence SAXS (GISAXS) geometry, and our method, and the average radius obtained by all the three techniques was almost the same for the two studied metals. Concerning the radius dispersion (RD), OES and GISAXS give very similar results, while TEM considerably underestimates the RD of the distribution.

Ablation and optical third-order nonlinearities in Ag nanoparticles.

The optical damage associated with high intensity laser excitation of silver nanoparticles (NPs) was studied. In order to investigate the mechanisms of optical nonlinearity of a nanocomposite and their relation with its ablation threshold, a high-purity silica sample implanted with Ag ions was exposed to different nanosecond and picosecond laser irradiations. The magnitude and sign of picosecond refractive and absorptive nonlinearities were measured near and far from the surface plasmon resonance (SPR) of the Ag NPs with a self-diffraction technique. Saturable optical absorption and electronic polarization related to self-focusing were identified. Linear absorption is the main process involved in nanosecond laser ablation, but non-linearities are important for ultrashort picosecond pulses when the absorptive process become significantly dependent on the irradiance. We estimated that near the resonance, picosecond intraband transitions allow an expanded distribution of energy among the NPs, in comparison to the energy distribution resulting in a case of far from resonance, when the most important absorption takes place in silica. We measured important differences in the ablation threshold and we estimated that the high selectiveness of the SPR of Ag NPs as well as their corresponding optical nonlinearities can be strongly significant for laser-induced controlled explosions, with potential applications for biomedical photothermal processes.

E′ and B2 center production in amorphous quartz by MeV Si and Au ion implantation

Abstract Amorphous SiO 2 was implanted with Si + and Au 2+ ions at energies of 4 and 3 MeV, respectively. The doses ranged from 0.3×10 17 to 1.5×10 17 ions cm −2 , and a current density of 30 μA cm −2 was used. The formation of E′ and B 2 centers was observed for both ions by optical absorption and electron paramagnetic resonance (for E′ defects) measurements. For Si implantation, there is a saturation of E′-center formation for doses higher than 1.0×10 17 Si cm −2 ; Au implantation produces E′-center annealing for sufficiently high doses. The heating effect due to the relatively high current density induces spontaneous formation of Au nanoparticles already during the ion bombardment. The effect of subsequent thermal annealing in a reducing atmosphere at 600 and 900°C on the E′ and B 2 centers was also studied.

Thermal spikes in Ag/Fe and Cu/Fe ion beam mixing

Abstract Ion beam mixing has been studied since 1980, and since then a lot of experimental and theoretical work has been done and knowledge has been gathered. Nevertheless, there are still many fundamental aspects that need to be clarified and with that aim many experiments need to be performed. Copper and iron are miscible in the liquid state, while silver and iron are not. However, both systems are thermally immiscible in the solid state. In order to have an insight into the importance of mixing within thermal spikes during ion beam irradiation, we deposited Cu/Fe and Ag/Fe bilayers onto Si substrates and irradiated them at room temperature with 2 MeV Cu and 2.5 MeV Au ions. A combination of Rutherford backscattering spectrometry (RBS) and atomic force microscopy (AFM) was used to analyze the atomic transport at the interface and the morphology changes of the samples. From the element profiles at the interface we conclude a mixing efficiency, which is indeed larger than the prediction of the ballistic model in the Cu/Fe system and smaller in the Ag/Fe system. Since ballistic mixing is expected in any case, we argue that demixing and phase separation in the Ag/Fe system occur in the thermal spike phase of the cascade as a consequence of the positive heat of mixing. Further mixing does occur in the thermal spike in the Cu/Fe system and they remain mixed even at the solid state because of the high cooling rate. In addition, ion irradiation induces a large surface roughening of the Ag and Cu top layers as proven by AFM. This effect is important for the correct interpretation of the results. Furthermore, this recrystallization affects also the interface, producing a rough interface, that appears in the RBS spectra as an atomic ‘diffusion’ at the interface.

RBS characterization of MgB2 superconducting films annealed ex situ and in situ

The elemental composition and depth profiles of MgB2 films prepared by successive e-beam evaporation as well as by thermal co-deposition of Mg and B components were investigated by Rutherford backscattering spectrometry (RBS). In the case of films deposited by e-beam evaporation we studied both Mg-B precursors and appropriate MgB2 films grown on glassy carbon, Si(100) and J-sapphire substrates annealed in situ. For the films co-deposited by thermal evaporation on R-sapphire substrates and annealed ex situ we investigated superconducting MgB2 films only. The Tco values of all MgB2 films ranged from 21 to 30 K. Because of a very fine granular structure of the annealed films, confirmed also by SEM observations, we could not identify any MgB2 phase from x-ray diffraction (XRD) patterns. On the other hand, Mg2Si phase has been detected by XRD on the film–substrate interface for the superconducting film deposited on Si(100) substrate. The RBS measurements were performed with a 3.1 MeV 4He+ beam. Under these conditions, the 16O(α,α)16 elastic resonance allowed us to detect oxygen in all studied samples especially in B layers. The depth profiles of precursors prepared by successive e-beam evaporation showed clearly the multilayer film structure consisting of B and Mg layers. A strong interdiffusion between B and Mg layers may be observed after an in situ annealing, but still some degree of non-homogeneous component distribution may be observed. On the other hand, the MgB2 films co-deposited by thermal evaporation and annealed ex situ are much more homogeneous, but a higher content of oxygen is present.

Ion beam analysis of HTc superconducting Tl-based films

Abstract The elemental composition, film thickness and concentration depth profiles of superconducting TlBaCaCuO and precursor BaCaCuO thin films were studied by IBA techniques such as RBS and NRA. The precursor BaCaCuO films were prepared by deposition of an aerosol (spray pyrolysis) atomized from aqueous nitrate solutions by ultrasonic excitations. The substrates were single crystal MgO and yttrium-stabilized zirconia (YSZ). The precursor BaCaCuO films were thallinated in a single-zone reaction chamber to produce the superconductor. The critical temperature values, Tc, of the superconducting films ranged from 101 to 103 K and were found to consist of more than 95 vol.% of the Tl2Ba2CaCu2Ox phase. IBA studies revealed that the superconducting films were well oxygenated but slightly thallium deficient, with the Tl depth profile decreasing from the bulk of the film to the surface. The phase composition was found to be different from the elemental one determined by IBA techniques. Moreover, residual carbon was found in both the superconducting and precursor films. Thermogravimetric studies revealed that it is highly probable that the carbon contamination was caused by exposure of the precursor oxide films to the ambient atmosphere prior to the thallination procedure. In regard to the optimization of the thallium content, the most important parameters of the dynamical thallination process were found to be the initial amount of Tl, the partial pressures pTl2O and pO2, the time of thallination and the reaction rate.

Formation of nanometer-scale structures in SiO2 thin films by means of MeV-ion irradiation

Future nanometer-scale electronic applications will require the engineering and modification of new materials on the atomic and molecular scale. In this article, we explore the possibility of using MeV heavy-ion irradiation to induce physical modifications on the materials of choice of the existing microelectronic technology, such as SiO2, metals and silicon, to form nanometer-scale structures. Micrometer-sized trenches or H-shaped structures were machined using focused ion-beam milling in Al/SiO2 and Au/SiO2 bilayers on crystalline Si. These samples were irradiated with a 4.6 MeV O2+ beam at room temperature, and the structures were studied by scanning electron microscopy. The micrometer-scale SiO2 structures underwent an anisotropic deformation induced by the electronic energy loss of the MeV incoming ions, leading to a major flow of SiO2 which fills the trench region. The target dimensions perpendicular to the beam direction increased, and the trench width or the micrometer-sized gaps of these structures decreased down to the nanometer scale as a function of the ion fluence. In the case of Al/SiO2 and Au/SiO2 bilayers, only the SiO2 film underwent the plastic deformation, and there is no flow of either the silicon substrate or the metallic layer.